Dehydrocyclodimerization is a reaction where reactants comprising paraffins and olefins containing from 2 to 5 carbon atoms per molecule are reacted in the presence of a catalyst to produce aromatics, with H.sub.2 and light ends as by-products. This process is quite different from the more conventional reforming or dehydrocyclization processes where C.sub.6 and higher carbon number reactants, primarily paraffins and naphthenes, are converted to aromatics. Aromatics formed in these conventional processes contain the same or a lesser number of carbon atoms per molecule as compared to the reactants from which they were formed, indicating the absence of dimerization reactions. In contrast, the dehydrocyclodimerization reaction results in an aromatic product that always contains more carbon atoms per molecule than the C.sub.2 to C.sub.5 reactants, thus indicating that the dimerization reaction is a primary step in the process of the present invention.
Typically, a dehydrocyclodimerization reaction is carried out at temperatures in excess of 500.degree. F. using dual-function catalysts containing an acidic component and a dehydrogenation component. These catalysts include acidic amorphous aluminas which contain metal promoters. Recently, crystalline aliminosilicates have been successfully employed as dehydrocylodimerization catalysts. Crystalline aluminosilicates, which are generally referred to as zeolites, may be represented by the empirical formula EQU M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O,
in which n is the valence of M, M is an element of Group I or Group II of the Periodic Table such as sodium, potassium, magnesium, calcium, strontium, or barium, and x is equal to or greater than 2.
Zeolites have skeletal structures which are made up of three dimensional networks of SiO.sub.4 and AiO.sub.4 tetrahedra, corner linked to each other by shared oxygen atoms. Such zeolites include mordenite and the ZSM variety. In addition to the zeolite component, certain metal promoters and inorganic oxide matrices have been included in dehydrocyclodimerization catalyst formulations. Examples of inorganic oxides include silica, alumina, and mixtures thereof. Metal promoters, such as Group VIII or Group III metals of the Periodic Table, have been used to provide the dehydrogenation functionality. The acidic function can be supplied by the inorganic oxide matrix, the zeolite, or both.
Molecular hydrogen is produced in a dehydrocyclodimerization reaction, as well as aromatic hydrocarbons. For example, reacting a C.sub.4 paraffin will yield 5 moles of hydrogen for every mole of aromatic produced. Because the equilibrium concentration of aromatics is inversely proportional to the fifth power of the hydrogen concentration, it is desired to carry out the reaction in the absence of added hydrogen. However, the absence of hydrogen promotes rapid catalyst deactivation, which is caused by carbon formation (coking) on the catalyst surface. This relatively rapid coke deposition makes it necessary to more frequently perform the costly and time-consuming catalyst regeneration procedure. Reducing catalyst coking, thereby increasing catalyst time in service before regeneration is necessary, is an object of this invention.
There are several basic process schemes by which catalyst may be regenerated. Catalyst in the reaction zone may be maintained in continuous use over an extended period of time, from about five months to about a year or more, depending on the quality of the catalyst and the nature of the feedstock. Following the extended period of operation, the reactor, or reactors, must be taken out of service while the catalyst is regenerated or replaced with fresh catalyst. Of course, this necessitates shutdown of the hydrocarbon conversion unit.
In another process scheme, known as the swing reactor method, catalyst is regenerated with greater frequency. A multiple fixed bed reactor system is arranged for serial flow of feedstock in such a manner that one reactor at a time can be taken off-stream while the catalyst in that reactor is regenerated or replaced with fresh catalyst. The reactor with fresh catalyst is placed on-stream when another reactor is taken off-stream for the catalyst bed to be regenerated or replaced with fresh catalyst.
In another process scheme, a moving bed reaction zone and regeneration zone are employed. Fresh catalyst particles are supplied to a reaction zone, which may be comprised of several sub-zones, and the particles flow through the zone by gravity. Catalyst is withdrawn from the bottom of the reaction zone and transported to a regeneration zone where a multi-step process is used to recondition the catalyst to restore its full reaction-promoting ability. Catalyst flows by gravity through the various regeneration steps and then is withdrawn from the regeneration zone and supplied to the reaction zone.
Movement of catalyst through the zones is often referred to as continuous though, in practice, it is semi-continuous. By semi-continuous movement is meant the repeated transfer of relatively small amounts of catalyst at closely spaced points in time. For example, one batch per minute may be withdrawn from the bottom of a reaction zone and withdrawal may take one-half minute, that is, catalyst will flow for one-half minute. If the inventory in the reaction zone is large, the catalyst bed may be considered to be continuously moving. This method of operation is preferred by many of those skilled in the art. When the moving bed method is used, there is no loss of production while catalyst is removed and replaced. Also, use of the moving bed method avoids the shutdown and startup procedures of the swing reactor system relating to insertion and removal of a reactor in the process stream.
Catalyst regeneration is preferably accomplished in a moving bed mode, where catalyst is passed through various treatment zones, rather than practicing the several regeneration stages in a fixed bed of catalyst. Catalyst is passed downwardly through a regeneration vessel by gravity, where it is contacted with a hot oxygen-containing gas stream (known as recycle gas) in order to remove coke which accumulates on surfaces of the catalyst while it is in a hydrocarbon conversion reaction zone. Coke is comprised primarily of carbon but is also comprised of a relatively small quantity of hydrogen. The mechanism of coke removal is oxidation to carbon monoxide, carbon dioxide, and water. The coke content of spent catalyst may be as much as 20% of the catalyst weight, though 5 to 7% is a more typical amount.
After passing through a combustion zone, catalyst is passed into a drying zone for removal of water formed in the combustion zone which has remained on the catalyst instead of being carried off with combustion gases. Water removal is accomplished by passing a hot dry air stream through the catalyst. In some cases, catalyst leaving a combustion zone is passed through a halogenation zone before it is dried. Catalyst is usually passed out of the regeneration vessel after drying is accomplished. It is then subjected to additional treatment steps in order to complete the total regeneration process.
A hot dry air stream is introduced into the bottom of the regeneration vessel and flows upward, countercurrent to catalyst flow. After passing through the catalyst drying zone to accomplish removal of water, the air stream passes into the gas collection portion of the combustion zone, where it mixes with gases produced by combustion and inert gases which have passed through the combustion zone catalyst. This mixture, termed flue gas, is withdrawn from the combustion zone and at least a portion of it is mixed with air and recycled back to the combustion zone to contact the catalyst to effect coke burn-off. The portion which is not recycled is simply vented to atmosphere. In an alternate method, the air stream leaving the drying zone will have a sufficient oxygen concentration, so that it is not necessary to add more air. Also, the air stream leaving the drying zone may first be passed, in whole or in part, through a halogenation zone.